Method for detecting the rotational position of the drive shaft a dc motor

ABSTRACT

The invention relates to a method for detecting the rotational position of the drive shaft of a DC motor and especially the position of an element driven by the drive shaft within a predetermined path of movement by evaluating the current ripples contained in the rotor current signal. The inventive method is characterized in that a period of a reference current ripple is provided during operation of the DC motor. The period of the reference current ripple is compared with the period of every current ripple detected during operation of the DC motor. If there is not sufficient agreement of the periods between the period of the reference current ripple and that of a detected current ripple, the periods of the current ripple signals that are not sufficiently in agreement are corrected with respect to the period of the reference current ripple.

[0001] The invention pertains to a method for determining the rotationalposition of the drive shaft of a direct current (DC) motor andespecially determining the position of an element driven by the driveshaft within a predetermined path of movement by evaluating the currentripples contained in the rotor current signal.

[0002] The rotor current signal of a DC motor is made up of a DCcomponent and an AC (analog current) component superimposed on the DCcomponent. The AC component results from the interaction of the magnet(field), the rotor windings, and the commutator of the DC motor duringoperation of the DC motor. This manifests itself through transientchanges in the induced voltage which produce ripples in the rotorcurrent signal. When the rotor is rotating the current spikes, called“current ripples” and “current peaks” in the following description, inthe rotor current signal occur at a frequency corresponding to thenumber of collector plates. If the rotor has, e.g., 10 collector plates,then 10 current ripples are detected in the rotor current signal. Thus,counting the current ripples provides information on the currentrotational position of the rotor of the DC motor and thus, providesinformation relative to the position of the driven element within itspredetermined path of movement. For this purpose, the analog rotorcurrent signal is digitized in order to count the current ripples.

[0003] Such a method is known, e.g., from DE 195 11 307 C1. So thatinterference pulses superimposed on the rotor current signal are notincluded in the evaluation of the current ripple count, typically theanalog rotor current signal undergoes conditioning before itsdigitization, e.g., it is subjected to frequency filtering. Thesepreviously known means and also other means are used so that a currentripple signal that is as free from interference pulses as much aspossible can be supplied for digitization and subsequent countevaluation. In this context, interference pulses do not include pulsesthat are conditional to the commutator and that are superimposed on therotor current signal.

[0004] However, during operation of a DC motor, particularly under aload, the current ripple contained in the rotor current signal canbecome distorted. Such a distortion can be evident by two current peaks.In the course of digitizing such a rotor current signal, instead of onecurrent peak, two current ripples are indicated at the position of thedistortion in the current ripple signal. Counting this double rippleleads to erroneous position determination of the driven element.Corresponding results apply for the absence or non-detection of acurrent ripple. These errors are conditional to the commutator and thusthey can be eliminated without anything further through conditioning ofthe analog rotor current signal.

[0005] Such methods are used, e.g., in the automotive field, forcontrolling an adjustable drive, like those provided, e.g., for windowsand sunroofs. An essential element in the determination of the position,e.g., of the window, is the element that can turn off the jammingprotection when the window is being closed. Turning off this protectionis necessary so that the window can travel completely into its upperblock and into the seals arranged in this block without the increasedload forcing the motor to be turned off. With erroneous counting of thecurrent ripples for determining the position of the window the jammingprotection may be turned off too early or too late.

[0006] Starting with the state of the art that has been discussed, theobjective of the invention is based on refining a method according tothe class mentioned above such that an exact current ripple count isguaranteed even when a current ripple contained in the rotor currentsignal is distorted or absent.

[0007] For the object of the invention, a correct, undistorted currentripple used as a calibration or reference variable is compared withother, possibly faulty current ripples. This alignment between acorrectly defined current ripple and the other ripples is done bycomparing each period with the others. For the reference current ripple,it is assumed that its period corresponds with sufficient precision tothe detected period of the current ripple in the rotor current signal ofthe DC motor or from the period determined from this signal. Acomparison of the periods of the reference current ripple with theperiod of each, actually measured current ripple in the current ripplesignal leads to the correction of periods in the current ripple signalwhich do not exhibit sufficient agreement with the period of thereference current ripple.

[0008] For providing a reference current ripple, in one embodiment, theDC motor operates in connection with a desired movement of the drivenelement from a first position into a second position for a short timeperiod without a load and the rotor current signal from this operatingstate of the DC motor is evaluated with reference to current ripplescontained in the signal. This configuration of the invention assumesthat erroneous current ripples which can be distorted or even missingoccur at a higher frequency when the DC motor operates under a load anderroneous ripples do not occur or only occur at a negligible frequencywhen the DC motor operates without a load. Thus, the periods of thecurrent ripple when the DC motor operates without a load can be used asreference current ripples.

[0009] The object of the invention uses the premise that a rapid changein the rotational speed of the DC motor, for instance a doubling or ahalving resulting in a corresponding halved or doubled period ascompared with the period of the reference current ripple, is notpossible due to the moment of inertia of the moving parts of the DCmotor. Conditional to the commutator, erroneous or distorted currentripples exhibit two maximums instead of one expected maximum in theanalog rotor current signal. For instance, the detection of a rapidhalving of the period for a current ripple relative to the period of thereference current ripple or also relative to the period of a previouscurrent ripple allows the conclusion to be made that this period halvingis not a result, for instance, of a rapid doubling of the rotationalspeed of the DC motor, but instead, it is a result of a distortedcurrent ripple. The number of current-ripple periods recognized aserroneous in the current ripple signal is then correctedcorrespondingly.

[0010] The period of the reference current ripple can be determinedadvantageously from the rpm (revolutions per minute) or speed of therotor of the DC motor. The rpm or speed of the rotor of the DC motor isdetermined from the present current and voltage values applied to the DCmotor. Because erroneous periods can be noticed either through doublingor halving of the period of the reference current ripple, the factorsthat influence the actual rpm or speed of the rotor of the DC motor inaddition to the current or voltage values, e.g., the motor inductivityor temperature influences, can be basically ignored. These factors canbe ignored because the variables associated with these factors do notchange enough to call attention to the fact that the period of thereference current ripple calculated from the detected rpm or speed isoff by a factor of two.

[0011] However, it is advantageous for the period of the referencecurrent ripple to have a tolerance range that is much smaller than thedetermined period of the reference current ripple. The period of thereference current ripple, with or without a tolerance range, can beadapted further depending on the actually measured period of theindividual current ripples, so that in this way, other factorsinfluencing the rpm and speed of the rotor are also considered inprocessing the period of the reference current ripple. This provides aself-teaching and somewhat self-calibrating system.

[0012] By comparing the period of a reference current ripple with eachperiod of the current ripples contained in the current ripple signal, acorrection is performed for each period recognized as erroneous withoutaveraging a period over a certain amount of time. Therefore, for themethod of the invention, the current ripple signal is corrected in itssmallest unit, namely the period of each current ripple. This has theresult that the rotational position of the drive shaft of the DC motorcan be determined in a cycle corresponding to the period of thereference current ripple.

[0013] The number of current-ripple periods recognized as defective inthe current ripple signal is corrected when a distorted current rippleoccurs. For instance, two current ripples are contained in the currentripple signal instead of only one, if the detected period is halvedrelative to the period of the reference current ripple; a correspondingcorrection is performed that divides the number of detected currentripples with halved periods by a factor of two. A corresponding processis performed for the inverse case when the period reproduced in thecurrent ripple signal is, e.g., doubled by an apparent fusion of twocurrent ripples. To correct this error, the number of such periods isdoubled.

[0014] Therefore, with the method according to the invention, an exactposition determination of the element driven by the DC motor isperformed even when faulty current ripples are contained in the currentripple signal due to certain load conditions.

[0015] Fundamentally, a correction as described above can be performedat an arbitrary time within each period, in which the DC motor isoperated for moving the driven element from a first position into asecond position. In the scope of the previously described embodiment, inwhich a reference current ripple is provided by motor operation withouta load, all that must be guaranteed for determining the referencecurrent ripple is that the motor operates for a period of time withoutor essentially without a load before performing the adjustment movementfor the driven element. This can be the case, e.g., at the motorstart-up phase or also at the motor stop-down phase. Such an operatingstate, where the DC motor driving the driven element operates without aload, can be realized through a correspondingly conceived mechanicalpower transmission means from the DC motor to the driven element, forinstance, a cable line. This can provide sufficient play in the powertransmission between the driving of DC motor and the driven element inorder to have a sufficient rotational movement of the DC motor without aload so that at least one reference current ripple can be detected.

[0016] The method according to the invention is suitable particularlyfor use in the scope of position detection for an adjustment device foropening and closing a window, a sunroof, or for adjusting a seat in amotor vehicle. Position determination, e.g., of the window, can then beperformed just by evaluating the current ripple signal without the useof additional sensors. However, with such a use, the reference currentripple must be detected at the motor start and thus before movement ofthe window, such that simultaneous with the movement, the desiredcorrection can be performed without having to store the entire currentripple signal at the beginning and without having to correct theperformed movement relative to the position determination of the drivenelement at the end. Only in this way can the corresponding position ofthe window be detected with sufficient reliability to indicate whetherthe jamming protection can be turned off depending on other factors sothat the window can be moved into the seals of the upper block.

[0017] Sketched in FIG. 1 is a diagram with an analog rotor currentsignal and current ripples contained in the signal, as well as thedigitized current ripple signal derived according to a known method. InFIG. 1, the first section on the left shows non-faulty current rippleswhich can serve as reference current ripples. In the adjacent section tothe right, the analog rotor current curve exhibits distorted currentripples. The distorted current ripples are reproduced in the digitizedcurrent ripple signal as so-called double ripples.

[0018]FIG. 2 shows a sketch of the course of the digitized currentripple curve before and after correction is performed according to theinvention. The lower curve, designated as “corrected”, represents areconstruction of the corrected current ripple signal. The correctionactually only performs a pulse count such that the curve designated ascorrected is the end result of the correction method. FIG. 2 clearlyshows that without correction, counting the current ripples indicatesthat the element driven by the DC motor has moved through a positionchange corresponding to the rotational angle value of ten collectorplates. However, the driven element has actually only been moved by arotational angle value corresponding to seven collector plates.

[0019] A plausibility check for the correction to be performed can bedone by comparing the period of one or more current ripples followingthe current ripple recognized as faulty. For this plausibility check,the assumption is used that distorted or missing current ripples in therotor current curve usually occur only for one collector plate and donot continue over several subsequent collector plates. However, with theobject of the invention, it is fundamentally possible to perform such aplausibility check for a certain number of periods.

[0020] For additional explanation of the invention, the embodiment shownin FIG. 3 is referenced. FIG. 3 shows a block circuit diagram of anarrangement for performing the method of the invention. The analog rotorcurrent signal of a DC motor 1 is detected to determine the rotationalposition of the rotor of the DC motor. A signal conditioning element 2conditions the analog rotor current signal to eliminate possible noisepulses. There can be different and also multiple signal processingsteps, e.g., a low-pass filter, to condition the rotor current signal.The prepared rotor current signal is applied to an A/D converter 3 fordigitizing the analog rotor current signal.

[0021] The present current and voltage values are simultaneously appliedto the input of a calculation element 4. Calculation element 4 is usedto provide the period of a reference current ripple. Calculation element4 determines the period from the current and voltage values applied tothe inputs of the calculation element and the resulting rpm. Inaddition, calculated periods are given a predetermined tolerance regionthat is much smaller than the calculated period.

[0022] A comparator 5 receives at its inputs both the output signal ofA/D converter 3 and also the reference period determined by calculationelement 4. Comparator 5 performs a comparison between the period of thereference current ripple and the period derived from the rotor currentsignal relative to a current ripple. For sufficient agreement of bothperiods, the actually measured period is fed to a current ripple counter6. For insufficient agreement of the two periods being compared, acorrection of the actually measured period is performed with referenceto the period made available by calculation element 4 as a referencevariable before this subsequently corrected period is fed to currentripple counter 6. The output of current ripple counter 6 is connected tothe input of an evaluation element (not shown in more detail) fordetermining the rotational position of the rotor of DC motor 1.

[0023] The signal at the output of comparator 5 is further connected toanother input of calculation element 4, such that in the calculation andconditioning of the reference current ripple, the periods, if necessarycorrected, of the preceding ripples can also be considered. In this way,the system illustrated in FIG. 3 is self-learning and can adapt tochanging influences. In particular, for this alignment of the actualperiods, the tolerance region of the reference periods can be made verysmall.

[0024] For the embodiment illustrated in FIG. 3, the individual elementsare illustrated as devices. However, the block circuit diagramillustrated in FIG. 3 can also be read as a flow chart, which makes itclear that the individual processing steps can also be realized insoftware. Summary of reference numbers 1 Motor 2 Signal conditioningelement 3 A/D converter 4 Calculation element 5 Comparator 6 Currentripple counter

1. Method for determining the rotational position of the drive shaft ofa DC motor by evaluating the current ripple contained in the rotorcurrent signal, characterized in that for an operation of the DC motor,a period of a reference current ripple is provided, the period of thereference current ripple is compared with the period of each currentripple detected during the operation of the DC motor, and for the casewhere the period of the reference current ripple does not sufficientlyagree with that of a detected current ripple, the periods of the currentripple signal, which are not in sufficient agreement, are corrected withreference to the period of the reference current ripple.
 2. Methodaccording to claim 1, characterized in that the period of the referencecurrent ripple is determined from the rpm of the rotor of the DC motor,which is itself derived from the present current and voltage valuesapplied to the DC motor.
 3. Method according to claim 2, characterizedin that the period of the reference current ripple includes a toleranceregion that is much smaller than the determined period of the referencecurrent ripple.
 4. Method according to claim 3, characterized in thatthe tolerance region is adapted depending on a period recognized aserror-free or a corrected period from the actual detected current ripplesignal.
 5. Method for determining the position of an element driven bythe drive shaft of a DC motor within a predetermined path of movement byevaluating the current ripple contained in the rotor current signal,characterized in that for an operation of the DC motor for moving thedriven element from a first position into a second position at least oneerror-free reference current ripple is detected, the period determinedfrom this reference current ripple is compared with the period of eachcurrent ripple detected during operation of the DC motor, and for thecase where the period of the reference current ripple does notsufficiently agree with that of a current ripple of the operating DCmotor, the periods of the current ripple signal of the operating DCmotor, which are not in sufficient agreement, are corrected withreference to the period of the reference current ripple.
 6. Methodaccording to claim 5, characterized in that for detecting the referencecurrent ripple of the DC motor, the motor operates essentially without aload for a certain amount of time before moving the driven element froma first position into a second.
 7. Method according to claim 6,characterized in that the DC motor is operated for a short time withouta load at its start or shortly after its start.
 8. Method according toone of claims 5-7, characterized in that a plausibility check of thecorrection of the period of the current ripple recognized as faulty isperformed using the period of the current ripple(s) following thecurrent ripple recognized as faulty.
 9. Use of the method according toone of claims 1-8 for position determination of a driven element in amotor vehicle, for instance, for position determination of a window, asunroof, or a seat.
 10. Device for determining the rotational positionof the drive shaft of a DC motor (1) by using the current ripplecontained in the rotor current signal with means (3) for digitizing theanalog rotor current signal and with a current ripple counter (6),characterized in that the device includes a calculation element (4) thatreceives as inputs the current and voltage values of the operating DCmotor (1), and outputs both from the means (3) for digitizing the analogrotor current signal and also from the calculation element (4) are inputto a comparator (5) for comparing a reference period provided by thecalculation element with each current ripple contained in the rotorcurrent signal and if necessary, for correcting the detected period,wherein the output of the comparator (5) is connected to the input ofthe current ripple counter (6).
 11. Device according to claim 10,characterized in that the signal at the output of the comparator (5) isalso applied to the input of the calculation element (4).